My Solution (so far):
The solution that I had decided up til now was that I will make several finite non-consumable items (like first-slot, second-slot ... etc). It's not the best solution.

Just recently I came to know about Firebase Realtime Database. I wanted to ask the experts if I could use Firebase Realtime Database for storing the slot counts the way I want, reliably, and without having the users to ever need to sign-up? I might be able to cater for the immediate and reliable retrieval of my data by using the

I contacted Firebase Support and it turns out that i CANNOT do that without implementing some sort of Account keeping per user.

Here is the actual reply:

I have been meaning to store count of a consumable in-app item for every user persistently.
Google play does not allow me to do that unless it is a non-consumable. I wanted to know if using firebase real-time database automatically keeps the data linked to the android users' google account?

Currently, there's no behavior of automatically handling the user
information or custom data associated with Google account when using
Firebase Realtime Database. You have to create your data linking
module associated with the user Google accounts and attach it with the
associated Firebase Realtime Database node of the user.

Is the data stored in the realtime database automatically per every user? If the user uninstall and then reinstalls, will the data remain?

You need to create your module in handling the data of the user using
Firebase Realtime Database or with your Server API. Note: Firebase
Realtime Database persistence/cache will be removed once the app is
uninstalled.